Some hydrogen generators generate hydrogen by reacting water vapor with a hydride fuel. The water vapor has been provided by different sources, such as ambient, a reservoir of water, or even as a byproduct of a chemical reaction such as in the case of fuel cells. When hydrogen is not required from the hydrogen generator, the supply of water vapor is shut off. The shut off has been accomplished by somewhat complex arrangements of valves.
Some arrangements of valves may be adversely affected by changes in ambient pressure, which may close a valve prematurely, resulting in a lack of water vapor where no reservoir is present. Such a change in ambient pressure may be caused by changes in elevation due to ascent on a passenger aircraft. Without a supply of water vapor, existing generated hydrogen may be consumed by a fuel cell, resulting in complete shutdown of the hydrogen generator and power generator incorporating the hydrogen generator and fuel cell until ambient pressure approaches its previous value.